NADH Oxidation Catalyzed by Electropolymerized Azines on Carbon Nanotube Modified Electrodes

Electron Transfer Between Enzymes and Electrodes

Efficient electron transfer between redox enzymes and electrocatalytic surfaces plays a significant role in development of novel energy conversion devices as well as novel reactors for production of commodities and fine chemicals. Major application examples are related to enzymatic fuel cells and electroenzymatic reactors, as well as enzymatic biosensors. The two former applications are still at the level of proof-of-concept, partly due to the low efficiency and obstacles to electron transfer between enzymes and electrodes. This chapter discusses the theoretical backgrounds of enzyme/electrode interactions, including the main mechanisms of electron transfer, as well as thermodynamic and kinetic aspects. Additionally, the main electrochemical methods of study are described for selected examples. Finally, some recent advancements in the preparation of enzyme-modified electrodes as well as electrodes for soluble co-factor regeneration are reviewed. Graphical Abstract.

Electrochemical sensors and biosensors based on redox polymer/carbon nanotube modified electrodes: a review

The aim of this review is to present the contributions to the development of electrochemical sensors and biosensors based on polyphenazine or polytriphenylmethane redox polymers together with carbon nanotubes (CNT) during recent years. Phenazine polymers have been widely used in analytical applications due to their inherent charge transport properties and electrocatalytic effects. At the same time, since the first report on a CNT-based sensor, their application in the electroanalytical chemistry field has demonstrated that the unique structure and properties of CNT are ideal for the design of electrochemical (bio)sensors. We describe here that the specific combination of phenazine/triphenylmethane polymers with CNT leads to an improved performance of the resulting sensing devices, because of their complementary electrical, electrochemical and mechanical properties, and also due to synergistic effects. The preparation of polymer/CNT modified electrodes will be presented together with their electrochemical and surface characterization, with emphasis on the contribution of each component on the overall properties of the modified electrodes. Their importance in analytical chemistry is demonstrated by the numerous applications based on polymer/CNT-driven electrocatalytic effects, and their analytical performance as (bio) sensors is discussed.

Facilitation of high-rate NADH electrocatalysis using electrochemically activated carbon materials

Electrochemical activation of glassy carbon, carbon paper and functionalized carbon nanotubes via high-applied-potential cyclic voltammetry leads to the formation of adsorbed, redox active functional groups and increased active surface area. Electrochemically activated carbon electrodes display enhanced activity toward nicotinamide adenine dinucleotide (NADH) oxidation, and more importantly, dramatically improved adsorption of bioelectrochemically active azine dyes. Adsorption of methylene green on an electroactivated carbon electrode yields a catalyst layer that is 1.8-fold more active toward NADH oxidation than an electrode prepared using electropolymerized methylene green. Stability studies using cyclic voltammetry indicate 70% activity retention after 4000 cycles. This work further facilitates the electrocatalysis of NADH oxidation for bioconversion, biosensor and bioenergy processes.

Characterization of Thermotoga maritima glycerol dehydrogenase for the enzymatic production of dihydroxyacetone

NAD-dependent Thermotoga maritima glycerol dehydrogenase (TmGlyDH) converts glycerol into dihydroxyacetone (DHA), a valuable synthetic precursor and sunless tanning agent. In this work, recombinant TmGlyDH was characterized to determine if it can be used to catalyze DHA production. The pH optima for glycerol oxidation and DHA reduction at 50 °C were 7.9 and 6.0, respectively. Under the conditions tested, TmGlyDH had a linear Arrhenius plot up to 80 °C. TmGlyDH was more thermostable than other glycerol dehydrogenases, remaining over 50 % active after 7 h at 50 °C. TmGlyDH was active on racemic 1,2-propanediol and produced (R)-1,2-propanediol from hydroxyacetone with an enantiomeric excess above 99 %, suggesting that TmGlyDH can also be used for chiral synthesis. (R)-1,2-propanediol production from hydroxyacetone was demonstrated for the first time in a one-enzyme cycling reaction using glycerol as the second substrate. Negative cooperativity was observed with glycerol and DHA, but not with the cofactor. Apparent kinetic parameters for glycerol, DHA, and NAD(H) were determined over a broad pH range. TmGlyDH showed little activity with N(6)-carboxymethyl-NAD(+) (N(6)-CM-NAD), an NAD(+) analog modified for easy immobilization to amino groups, but the double mutation V44A/K157G increased catalytic efficiency with N(6)-CM-NAD(+) ten-fold. Finally, we showed for the first time that a GlyDH is active with immobilized N(6)-CM-NAD(+), suggesting that N(6)-CM-NAD(+) can be immobilized on an electrode to allow TmGlyDH activity in a system that reoxidizes the cofactor electrocatalytically.

Electrochemical oxidation of dihydronicotinamide adenine dinucleotide at nitrogen-doped carbon nanotube electrodes

Nitrogen-doped carbon nanotubes (N-CNTs) substantially lower the overpotential necessary for dihydronicotinamide adenine dinucleotide (NADH) oxidation compared to nondoped CNTs or traditional carbon electrodes such as glassy carbon (GC). We observe a 370 mV shift in the peak potential (Ep) from GC to CNTs and another 170 mV shift from CNTs to 7.4 atom % N-CNTs in a sodium phosphate buffer solution (pH 7.0) with 2.0 mM NADH (scan rate 10 mV/s). The sensitivity of 7.4 atom % N-CNTs to NADH was measured at 0.30 ± 0.04 A M(-1) cm(-2), with a limit of detection at 1.1 ± 0.3 μM and a linear range of 70 ± 10 μM poised at a low potential of -0.32 V (vs Hg/Hg2SO4). NADH fouling, known to occur to the electrode surface during NADH oxidation, was investigated by measuring both the change in Ep and the resulting loss of electrode sensitivity. NADH degradation, known to occur in phosphate buffer, was characterized by absorbance at 340 nm and correlated with the loss of NADH electroactivity. N-CNTs are further demonstrated to be an effective platform for dehydrogenase-based biosensing by allowing glucose dehydrogenase to spontaneously adsorb onto the N-CNT surface and measuring the resulting electrode's sensitivity to glucose. The glucose biosensor had a sensitivity of 0.032 ± 0.003 A M(-1) cm(-2), a limit of detection at 6 ± 1 μM, and a linear range of 440 ± 50 μM.

Effect of surfactant type and redox polymer type on single-walled carbon nanotube modified electrodes

Electrodes modified with single-walled carbon nanotubes (SWNTs) offer a number of attractive properties for developing novel electrochemical sensors. A common method to immobilize SWNTs onto the electrode surface is by placing a droplet of a SWNT suspension onto the electrode surface and allowing the solvent to evaporate. In order to maximize the properties of individual SWNTs, surfactants are normally present in these suspensions to provide stable and homogeneous SWNT dispersions. In this study we investigated the effect of different surfactants on the electrochemical and enzymatic performance of SWNT modified glassy carbon electrodes (GCEs). Amperometic biosensors for glucose were fabricated by a two-step procedure. In the first step, SWNT films were deposited onto GCEs by solution casting suspensions of SWNTs in water, Triton X-100, Tween 20, sodium cholate or sodium dodecylbenzenesulfonate (NaDDBS). In the second step, hydrogels containing a redox polymer and the enzyme, glucose oxidase (GOX), were deposited and cross-linked onto the SWNT-modified GCE. Three different redox polymers were tested: 3-ferrocenylpropyl-modified LPEI, (Fc-C3-LPEI), 6-ferrocenylhexyl-modified LPEI, (Fc-C6-LPEI), and poly[(vinylpyridine)Os(bipyridyl)2Cl](2+/3+)(PVP-Os). Biosensors constructed with SWNT films from suspensions of Triton X-100 or Tween 20 generally produced the highest electrochemical and enzymatic responses, with Triton X-100 films producing current densities of ~1.7-2.1 mA/cm(2) for the three different redox polymers. In contrast, biosensors constructed with SWNT films from sodium cholate suspensions resulted in significant decreases in the electrochemical and enzymatic response and in some cases showed no enzymatic activity. The results with SWNT films from NaDDBS suspensions were dependent upon the specific redox polymer used, but in general gave reduced enzymatic responses (~0.05-0.4 mA/cm(2)). These results demonstrate the importance of surfactant type in fabricating SWNT-modified electrode films.

One-step fabrication of integrated disposable biosensor based on ADH/NAD+/meldola's blue/graphitized mesoporous carbons/chitosan nanobiocomposite for ethanol detection

A novel strategy to simplify the dehydrogenase-based electrochemical biosensor fabrication through one-step drop-coating nanobiocomposite on a screen printed electrode (SPE) was developed. The nanobiocomposite was prepared by successively adding graphitized mesoporous carbons (GMCs), meldola's blue (MDB), alcohol dehydrogenase (ADH) and cofactor nicotinamide adenine dinucleotide (NAD(+)) in chitosan (CS) solution. MDB/GMCs/CS film was prepared. Cyclic voltammetry measurements demonstrated that MDB was strongly adsorbed on GMCs. After optimizing the concentration of MDB and the working potential, the MDB/GMCs/CS film presented a fast amperometric response (5s), excellent sensitivity (10.36 nA μM(-1)), wide linear range (10-410 μM) toward NADH and without any other interference signals (such as AA, UA, DA, H2O2 and metal ions). Furthermore, concentrations of ADH and NAD(+) in nanobiocomposite and the detection conditions (temperature and pH) were also optimized. The constructed disposable ethanol biosensor showed an excellent linear response ranged from 0.5 to 15 mM with high sensitivity (67.28 nA mM(-1)) and a low limit of detection (80 μM) and a remarkable long-term stability (40 days). The intra-batch and inter-batch variation coefficients were both less than 5% (n=5). The ethanol recovery test demonstrated that the proposed biosensor offered a remarkable and accurate method for ethanol detection in the real blood samples.

Reagentless D-sorbitol biosensor based on D-sorbitol dehydrogenase immobilized in a sol-gel carbon nanotubes-poly(methylene green) composite

A reagentless D-sorbitol biosensor based on NAD-dependent D-sorbitol dehydrogenase (DSDH) immobilized in a sol-gel carbon nanotubes-poly(methylene green) composite has been developed. It was prepared by durably immobilizing the NAD(+) cofactor with DSDH in a sol-gel thin film on the surface of carbon nanotubes functionalized with poly(methylene green). This device enables selective determination of D-sorbitol at 0.2 V with a sensitivity of 8.7 μA mmol(-1) L cm(-2) and a detection limit of 0.11 mmol L(-1). Moreover, this biosensor has excellent operational stability upon continuous use in hydrodynamic conditions.

Electrochemistry of Q-graphene

A newly synthesised type of graphene, Q-Graphene, has been physically and electrochemically characterised with Scanning and Transmission Electron Microscopy (SEM, TEM), X-ray Photoelectron Spectroscopy (XPS) and Cyclic Voltammetry (CV). Interpretation of SEM, TEM and XPS data reveal the material to consist of hollow carbon nanospheres of multi-layer graphene (viz. graphite), which exhibit a total oxygen content of ca. 36.0% (atomic weight via XPS). In addition to the carbon structures present, spherical magnesium oxide particles of ≤50 nm in diameter are abundantly present in the sample (ca. 16.2%). Interestingly, although the TEM/SEM images show macroporous carbon structures, Raman spectroscopy shows peaks typically characteristic of graphene, which suggests the material is highly heterogeneous and consists of many types of carbon allotropes. Q-Graphene is electrochemically characterised using both inner-sphere and outer-sphere electrochemical redox probes, namely potassium ferrocyanide(II), hexaammine-ruthenium(III) chloride and hexachloroiridate(III), in addition to the biologically relevant and electroactive analytes, norepinephrine, β-nicotinamide adenine dinucleotide (NADH) and l-ascorbic acid. The electrochemical response of Q-Graphene is benchmarked against edge plane- and basal plane-pyrolytic graphite (EPPG and BPPG respectively), pristine graphene and graphite alternatives. Q-Graphene is found to exhibit fast electron transfer kinetics, likely due to its high proportion of folded edges and surface defects, exhibiting a response similar to that of EPPG - which exhibits fast electron transfer rates due to the high proportion of edge plane sites it possesses. Furthermore, we demonstrate that the specific oxygen content plays a pivotal role in dictating the observed electrochemical response, which is analyte dependant. Consequently there is potential for this new member of the graphene family to be beneficially utilised in various electrochemical applications.